1. Field of the Invention
The present invention relates to a PWM signal generator and a PWM signal generating method for generating a pulse width modulation (PWM) signal in accordance with a pulse code modulation (PCM) digital signal.
2. Description of the Related Background Art
FIG. 1 shows the configuration of a digital-to-analog converter to which a conventional PWM signal generator is applied. The digital-to-analog converter comprises a digital interface 1, an oversampling circuit 2, a delta-sigma modulator 3, a PCM-PWM converter 4, a differential amplifier 5, and a low pass filter 6.
FIG. 2 shows the configuration of a conventional digital amplifier to which a conventional PWM signal generator is applied. The digital amplifier comprises a digital interface 11, an oversampling circuit 12, a digital volume 13, a delta-signal modulator 14, a PCM-PWM converter 15, a power switching circuit 16, and a low pass filter 17.
The digital amplifier 2 in FIG. 2 differs from the digital-to-analog converter illustrated in FIG. 1 only in that the digital volume 13 is provided, and the differential amplifier 5 is replaced by the power switching circuit 16, and is identical in the rest, so that the digital amplifier in FIG. 2 will be next described.
A digital signal read from a disk such as CD, DVD or the like is supplied into the digital amplifier through the digital interface 11, and the sampling frequency of the digital signal is increased by the oversampling circuit 12. Subsequently, the gain of the digital signal is adjusted by the digital volume 13.
The delta-sigma modulator 14 is for example, a noise shaper having subtractors 21, 23, a quantizer 22, and a filter 24, as shown in FIG. 3, and reduces the number of quantization bits of the gain adjusted digital signal. In the delta-signal modulator 14, quantization noise Nq(z) output from the subtractor 23 is fed back to the subtractor 21 on the input side through the filter 24 (transfer function H(z)). As a result, the spectrum of the quantization noise is converted to N′q(z)=[1−H(z)]Nq(z). By adjusting the transfer function H(z) of the filter 24, the quantization noise can be shifted to a higher frequency band to ensure the S/N ratio for an audio band, as shown in FIG. 4A. FIG. 4B is an enlarged view of the frequency spectrum in FIG. 4A.
The output digital signal of the delta-sigma modulator 14 is converted to two types of PWM signals by the PCM-PWM converter 15. The PWM signals consist of PWM—A and PWM—B which are supplied to the power switching circuit 16. The power switching circuit 16 converts the PWM signals PWM—A and PWM—B to a PWM differential signal indicating a differential component of PWM—A−PWM—B. The power switching circuit 16 includes an H-bridge switching circuit having four switching elements SW1–SW4, for example, as shown in FIG. 5. The differential signal PWM—A−PWM—B is supplied to a speaker 18 through the low pass filter 17. The low pass filter 17 supplies the speaker 18 with an audio band signal which is a lower band component of the differential signal PWM—A−PWM—B.
However, since digital PWM is essentially non-linear processing, this causes harmonic distortion and intermodulation distortion in signal components, and causes an intermodulation distortion in a quantization noise component. The intermodulation distortion component of the quantization noise shifted to a higher band by the noise shaping drop down to the audio band to raise the noise floor of the audio band. This exacerbates the harmonic distortion characteristic and S/N ratio of the PWM output as compared with the output of the delta-sigma modulator.
FIG. 6 shows a method of generating a single sided binary PWM signal. A PWM signal PWM—A having a pulse width in accordance with a PCM signal, and PWM—B which is a NOT signal of PWM—A are generated. A difference PWM—A−PWM—B of the PWM signals PWM—A and PWM—B is a PWM differential signal. The frequency spectrum of the PWM differential signal is as shown in FIG. 7, where a harmonic distortion is generated and the noise floor rises, as compared with the output of the delta-sigma modulator shown in FIG. 4.
Single-sided three-valued PWM, or double sided three-valued PWM is known as a prior art technique for reducing the harmonic distortion and lowering the noise floor. A method of generating a single sided three-valued PWM signal is performed as shown in FIG. 8. PWM signals PWM—A which has a pulse width in accordance with a PCM signal, and PWM—B which is 2's complement signal of PWM—A are generated. The difference PWM—A−PWM—B between the PWM signals PWM—A and PWM—B is a PWM differential signal. The frequency spectrum of the PWM differential signal is as shown in FIG. 9, where even-number order harmonic distortions disappear and the noise floor is lowered.
A method of generating a double sided three-valued PWM signal is performed as shown in FIG. 10. PWM signals PWM—A which has a pulse width in accordance with a PCM signal, and PWM—B which is a 2's complement signal of PWM—A are generated to be symmetric with respect to the center (four in this case). The difference PWM—A−PWM—B between the PWM signals PWM—A and PWM—B is a final PWM output signal. The frequency spectrum of the PWM output signal is as shown in FIG. 11, where even-number order harmonic distortions disappear and the noise floor is lowered further as compared with the single sided three-valued PWM.
As described above, when the single-sided three-valued PWM is employed for reducing the harmonic distortion and lowering the noise floor, even-number order harmonic distortions can be removed in principle. However, a problem lies in that odd-number order harmonic distortions still remain large. The double-sided three-valued PWM in turn provides an ideal modulation which removes the even-number order harmonic distortions, reduces the odd-number order harmonic distortions, and lowers the noise floor. However, since PWM—A and PWM—B must be generated to be symmetric with respect to the center even for odd-number PCM signals, a problem lies in that the clock frequency must be twice as high, and the configuration is complicated as compared with the single-sided PWM.
It is therefore an object of the present invention to provide a PWM signal generator and generating method which are capable of reducing both harmonic distortion and noise floor without doubling the clock frequency, and a digital-to-analog converter and a digital amplifier to which the PWM signal generator is applied.